The main organs of digestion—namely, the colon or large intestine, the small intestine or small bowel, and the rectum—sit together, functioning in continuity, in the crowded abdomino-pelvic cavity. And endometriosis can have a massively disruptive impact on this area. 

Unlike the cells of the uterine lining, which flow out of the body during menstruation, endo cells do not dissipate through a natural exit; rather, they thicken and expand. As they do, these endo implants (i.e., the clumps of tissue that grow outside your uterus) affect both the organs they are growing on or adjacent to and the muscles and fascia that underlie the organs. Their presence can make organs adhere to one another or tense and tighten disproportionately to one side. 

The anatomical distortion caused by endo implants on or near the GI tract can trigger symptoms like diarrhea, constipation, bloating, painful bowel movements, and abdominal pain, all of which can be incredibly uncomfortable and unpredictable. The pulling and tugging on the fascia and muscles can prevent the bowel's normal peristalsis (the contraction and relaxation that prompts you to poop) and can slow its motility, causing constipation. Sometimes, the constipation becomes so bad that women are only able to release the rare watery stool, with some patients even needing to use their finger to "trigger" a bowel movement.  

Meanwhile, the inflammation of endometriosis plus the slowdown of bowel motility mean that the bacteria of the small intestine stay there for a long time, during which they fester and grow well beyond what is normal. That condition is called small intestinal bacterial overgrowth (SIBO), and it can impede the absorption of nutrients into the bloodstream and release enzymes and gases that cause bloat and pain as well as diarrhea. 

In some women, these symptoms occur at random. In others, they occur in sync with a woman's cycle—after being constipated for two or three weeks, the arrival of the menstrual flow metaphorically pops a cork in these women, and liquid stool just pours out of them. And then the cycle begins again. And even if these events occur only sporadically, the fact of their recurrence is a signal that endo could well be the cause.

Additionally, pelvic and abdominal muscle aches from straining in the bathroom, phantom urges to poop, and any lower GI pain that feels disproportionately severe can also all be indicators you may be dealing with endometriosis.